1. Field of the Invention
The present invention is concerned with an improved system for latent or invisible imaging of printed material onto a variety of substrates, with subsequent development of the latent image by application of a corresponding developing fluid. More particularly, it is concerned with such a latent imaging system, and a developer forming a part thereof, which makes use of a nickel salt in a non-blurring, non-sublimating, stable, invisible imaging ink, and a color precursor in the developer fluid which includes a dioxime compound capable of complexing with nickel cation to give a sharp, long lasting, non-smearing color. The developer advantageously includes a hydroxide accelerator for accelerating the reaction between the dioxime compound and the nickel ion.
2. Description of the Prior Art
Modern day sympathetic ink systems of commercial importance are designed for a variety of uses, for example in self-testing educational booklets. In such uses, a student may be provided with a multiple choice answer sheet, with the correct answer being indicated by appropriate latent image indicia. In selecting an answer, the student marks one of the multiple choice possibilities with developer substance; if his selection is correct, the latent indicia will reveal the student's correct choice and hence his progress. Other uses include certification testing, personnel selection and screening tests, novelty items such as games and toys, children's books designed for educational and/or entertainment purposes, securing of documents, prize verification and promotional items.
Systems of the type described typically involve application of a latent image by means of a normally invisible printing ink. Thereafter, the developer substance is applied over the invisible image and a color-forming chemical reaction occurs to "develop" the image and render the same visible. For example, U.S. Pat. No. 3,632,364 describes a latent imaging system wherein the invisible ink includes a soluble copper salt such as copper sulfate, while the developer includes a soluble iodide. In practice, the invisible ink is printed onto conventional paper, and is invisible to the naked eye. When the developer is applied, the iodide is oxidized by the copper ion present in the printed image to release iodine. The iodine in turn reacts with the starch in the paper to form a color. Other patents describing various latent imaging systems include U.S. Pat. Nos. 3,823,022, 4,051,283, 3,788,863, 3,451,143, 3,363,338, 3,349,408 and 4,631,203. Other references include British Pat. No. 415,535 and an article entitled "Tintas Simpaticas" by P. L. De Araujo Feio, Revista da Sociedade Brasileira de Quimica, Boletin Cientifico, Vol. XIV, Janeiro Marco de 1945, N. 1, Rio de Janeiro, Brazil. While certain latent imaging systems of the type described have commercial application in, e.g., education self-testing materials and the like, they are typically plagued by a number deficiencies. For example, the copper sulfate/soluble iodide system tends to develop an unstable color which will disappear under prolonged heating or ultraviolet light conditions. Moreover, in many instances the visual image lacks sharpness and clarity, and the color can smear if repeated applications of the developing fluid are made. Furthermore, use of this type of system is in practice generally limited to paper substrates, because of the necessity of having starch present as a part of the overall reaction. Thus, such a system is much more difficult to employ in the case of non-paper substrates such as metal or synthetic resin bodies.
The aforementioned article entitled "Tintas Simpaticas" also teaches the use of "dimetilglioxima a 1%" as the "elemento principal da T. S." in conjunction with "sulfato de niquel" as the "revelador" thereby yielding "rosa" as the "coloracao que toma a T. S. ao ser revelada." It is therefore believed that the article suggests the use of dimethylglyoxime at 1% as a latent imaging ink to be applied to an appropriate substrate and nickel sulfate as the developer ultimately applied over the dimethylglyoxime ink to yield a red colored print. This disclosed method of latent imaging has several serious problems associated with it. For example, when dimethylglyoxime is used as an ink applied to a paper substrate it tends to sublimate, thereby transferring ink between adjacent pages. In the case of self-testing booklets for example this can be a problem because imprinted material can be transferred page-to-page making the booklet very confusing to the user. Moreover, dimethylglyoxime also tends to migrate within a paper page and thereby blur the printed images. This in turn makes it difficult to employ fine printing with such inks. As a consequence of these problems there are no known instances of commercial use of a dimethylglyoxime ink/nickel sulfate developer latent imaging system.